This invention relates generally to shaft sealing devices for use with rotating equipment. Adequate maintenance of rotating equipment is difficult to obtain because of extreme equipment duty cycles, the lessening of service factors, design, and the lack of spare rotating equipment in most processing plants. This is especially true of machine tool spindles, wet-end paper machine rolls, aluminum rolling mills, and steam quench pumps and other equipment utilizing extreme contamination affecting lubrication. Various forms of shaft sealing devices have been utilized to try to protect the integrity of the bearing environment, including rubber lip seals, clearance labyrinth seals, and attraction magnetic seals. Lip seals or other contacting shaft seals can quickly wear to failure and are also known to permit excessive amounts of moisture and other contaminants to immigrate into the oil reservoir of the operating equipment before seal failure had exposed the interface between the rotor and the stator to the contaminants or lubricants at the radial extremity of the seal. The problem of seal wear and damage as applied to electrical motors using variable frequency drives is compounded because of the very nature of the control of electricity connected to variable frequency drive (VFD) controlled motors.
VFDs regulate the speed of a motor by converting sinusoidal line alternating current (AC) voltage to direct current (DC) voltage, then back to a pulse width modulated (PWM) AC voltage of variable frequency. The switching frequency of these pulses ranges from 1 kHz up to 20 kHz and is referred to as the “carrier frequency.” The ratio of change in voltage to the change in time (ΔV/ΔT) creates what has been described as a parasitic capacitance between the motor stator and the rotor, which induces a voltage on the rotor shaft. If the voltage induced on the shaft, which is referred to as “common mode voltage” or “shaft voltage,” builds up to a sufficient level, it can discharge to ground through the bearings. Current that finds its way to ground through the motor bearings in this manner is called “bearing current.”1 1 http://www.greenheck.com/technical/tech_detail.php?display=files/Product_guide/fall7_03
There are many causes of bearing current including voltage pulse overshoot in the VFD, non-symmetry of the motor's magnetic circuit, supply imbalances, and transient conditions, among other causes. Any of these conditions can occur independently or simultaneously to create bearing currents in the motor shaft.2 2 http://www.greenheck.com/technical/tech_detail.php?display=files/Product_guide/fall7_03
Shaft voltage accumulates on the rotor until it exceeds the dielectric capacity of the motor bearing lubricant, then the voltage discharges in a short pulse to ground through the bearing. After discharge, the voltage again accumulates on the shaft and the cycle repeats itself. This random and frequent discharging has an electric discharge machining (EDM) effect, causing pitting of the bearing's rolling elements and raceways. Initially, these discharges create a “frosted” or “sandblasted” effect. Over time, this deterioration causes a groove pattern in the bearing race called “fluting,” which is an indication that the bearing has sustained severe damage. Eventually, the deterioration will lead to complete bearing failure.3 3 See www.Greenheck.com
The prior art teaches numerous methods of mitigating the damage shaft voltages cause, including using a shielded cable, grounding the shaft, insulated bearings, and installation of a Faraday shield. For example, U.S. Pat. No. 7,193,836 discloses devices for controlling shaft current, which devices are designed to induce ionization in the presence of an electrical field.
Most external applications add to costs, complexity, and exposure to external environmental factors. Insulated bearings provide an internal solution by eliminating the path to ground through the bearing for current to flow. However, installing insulated bearings does not eliminate the shaft voltage, which will still find the lowest impedance path to ground. Thus, insulated bearings are not effective if the impedance path is through the driven load. Therefore, the prior art does not teach an internal, low wearing method or apparatus to efficaciously ground shaft voltage and avoid electric discharge machining of bearings leading to premature bearing failure.